1. Field of the Invention
This invention relates generally to the measurement of mechanical strain associated with electrode reactions and, more particularly, to strain measurement during materials deposition, chemical reactions and relaxation in thin films on fine optical fibers.
2. History of the Prior Art
The processes of manufacturing microcircuits, sensitive measurement devices and various elements of computer hardware often include a step in which one or more thin films of a metal are electrodeposited onto a substrate. One of the most important factors influencing the behavior of such electrodeposited films is the strain induced by the electrodepositing process itself.
Few means are available for observing the physical structure of very thin electrodeposits. For example, the electron microscope, which has been used extensively for research on evaporated films, is of rather limited utility because its use requires rather difficult specimen preparation, and such an instrument is not usable to make dynamic measurements during the course of electrodeposition of a film. One approach, as disclosed in "Measurement of Stress in Very Thin Electrodeposits", by H. Watkins and A. Kolk, Journal of the Electrochemical Society, November 1961, pages 1018-1023, suggests that combining stress measurements with electron micrographs may permit better understanding of the structural detail in very thin film regions and that such stress data may be utilized to indicate the beginning of film continuity and to indicate structural change as the electroplate thickness increases. Watkins et al describe a modified form of the Brenner-Senderoff contractometer that provides greater sensitivity through the use of jeweled bearings and optical readout for determining quantitive stress data in films as thin as 40 A average thickness, with electron micrographs of the film made to help in interpreting the stress data.
It should also be appreciated that certain types of very precise sensors and instrumentation employ optical fibers coated with very thin metallic films to make physical or chemical measurements. An example of such a device is disclosed in "Optical Fiber Hydrogen Sensor", by M. A. Butler, Appl. Phys. Lett., 45 (10), Nov. 15, 1984, wherein an optical fiber is coated with palladium which expands on exposure to hydrogen to change the effective optical path length of the fiber, this change being detected by interferometric techniques. Earlier experiments have demonstrated this effect and suggest a high sensitivity and a wide dynamic range for this kind of sensor.
U.S. Pat. No. 4,092,849, to Maxwell, discloses an apparatus and a method for measuring the elastic properties of polymer melts and polymer solutions, including measurement of the force required to shear a specimen, in order to determine the modulus of elasticity, stress and steady state viscosity of the specimen. In this apparatus, a specimen of the material to be tested is placed within an intervening space between two coaxial members, both of which are capable of low friction rotation about a common axis so that one member is forcibly rotated with respect to the other to shear the specimen. The member that is not forceably rotated is then released, and measurements of the recoverable strain and rate of strain recovery are made by measuring the motion of the released member as a function of time, with amplification of the motion provided by conveying light along a length of optical fiber disposed substantially normal to the axis of the moving member. Pulses of light are periodically provided into a portion of the optical fiber aligned with the axis, to be conveyed along the length of the optical fiber radially outward from the axis and through a small portion aligned parallel to the axis for projection onto a photographic film to record the changing position over time of the light projected from the optical fiber end.
Devices of the type discussed hereinabove, while each serving a specific need, do not provide or suggest a satisfactory solution to the need for information on the strain generated during the process of electrodeposition in a thin film, chemical, biological or physical reactions in or with such a thin film and relaxation within the structure of a thin film during, for example, a hiatus in the electrodeposition process. A need, therefore, clearly exists for a sensitive, versatile, and reliable apparatus and method for determining strain and rate of strain as functions of time in very thin films being deposited on, located on, or being removed from, substrates such as very thin optical fibers.